Yes, I've been battling this one for few days now and it appears that the advanced search facility was removed from Profile Manager, which in my eyes is a big boo as that is one of the reasons why I started using Elgg! But apparently something is in development.

The advanced members search was part of profile manager, but was removed for the 1.8 compatible version. I believe the ColdTrick guys intended to add it as a separate advanced search plugin but it never came to fruition.

i thought about making a multi-tag search function as part of the related-items plugin and briefly looked into that.. however, i decided to code a custom page instead (which is technically similar, yes).

i want to extend the related-items plugin so that the 'view all related items' page has a sidebar which allows filtering via subtype, so that would essentially be a multi-tag search page that accepts entity object guids instead of searching on a manually supplied/typed list of tags.

so ultimately the multi-tag search function would be the preferred route here for me too since the related-items plugin can then make use of the search page instead of using its own custom page.